1. Field of the Invention
The present invention relates to methods for the improvement of mechanical properties of aluminum can body stock and end stock material. More particularly, the present invention relates to methods for improving the yield strength and in some cases the yield strength and earing of aluminum can body stock and end stock alloys.
2. Description of the Prior Art
The materials most commonly used in the manufacture of drawn and ironed beverage containers are the Aluminum Association Specification AA 3XXX (where X represents an integer from zero to nine) series of aluminum alloys. This series of alloys is known as the AA 3000 series of alloys. The alloys in this series contain manganese and are strengthened primarily by the formation of second phase precipitate particles.
The materials most commonly used in the manufacture of metal beverage container ends and closures are the Aluminum Association Specification AA 5XXX (where X represents an integer from zero to nine) series of aluminum alloys. This series of alloys is known as the AA 5000 series of alloys. This series of alloys is characterized by a solid solution of alloying elements (primarily magnesium) which confers a strength higher than that of unalloyed aluminum. Alloys of this series are, in general, stronger but less formable than those of the AA 3000 series and generally exhibit higher work-hardening rates.
The AA 3000 series of aluminum alloys is of considerable economic importance in the metal beverage container packaging industry. For instance, in 1988, 3.7 billion pounds of the AA 3004 aluminum alloy, a member of the AA 3000 series, were used in metal beverage container production. This use represents the largest single use of aluminum and its alloys. Increased demand from the metal beverage container packaging industry for aluminum cans has created a considerable need for aluminum alloy sheet material for forming the can body and end portions that is economical to manufacture and possesses a combination of desirable formability and strength properties. Thus, it would be quite advantageous to produce aluminum alloy sheet material having improved yield strength and in some cases improved yield strength and improved earing.
According to conventional processes for producing aluminum alloy sheet material that is subsequently deep drawn and ironed into beverage cans, the aluminum alloy material is initially cast by strip or direct chill casting processes into an ingot having a thickness of about 20-30 inches. The ingot is then homogenized by a two step process, in which the ingot is first heated at a temperature of 1125.degree. F. for four hours and is then heated at a temperature of 975.degree. F. for 2 hours. The homogenized ingot is then hot rolled to a thickness of from 0.080 to 0.130 inches to form the hot band material. Next, the hot band material is annealed at a temperature of from 600.degree. to 900.degree. F. to effect softening and recrystallization of the aluminum alloy material. The material is then cold rolled 80-90% to its final thickness to produce material having a super hard temper known as the Aluminum Association Specification H19 temper.
The major mechanical properties of the AA 3000 series of aluminum alloys such as the AA 3004 alloy in the H 19 condition are a yield strength after baking of about 35 ksi and earing of about 2.0%.
The present invention has been developed with a view to providing processes for producing aluminum alloy sheet material having improved mechanical properties.